About IP addresses

When a visitor posts as Guest, their IP address still shows up to the Admin, as it should.  What I want to do is look up this IP address in the forum database and see if I can tell who it is -- maybe a forum member who is wanting to post anonymously, perhaps. 

Is there a table in the database that has IP address stored, or do I have to look through each topic in each forum for a post with a member name and this IP to get a match?  Or is there a better way to do this?